IGCSE Biology: Organisms and their Environment Practice Questions
Almost all energy in ecosystems originates from the Sun and enters food chains through photosynthesis in producers. Energy is lost at every trophic level, mainly through respiration, movement and excretion, which is why food chains rarely have more than four or five levels.
Topic 19 of Cambridge IGCSE Biology 0610 is a data interpretation topic as much as a recall topic. Energy loss between trophic levels is the explanation Cambridge asks for most often, and it needs specific named routes rather than the phrase energy is lost. The questions below cover energy flow, pyramids, the carbon cycle and population curves.
What you need to know for Organisms and their Environment
- Trophic levelThe position of an organism in a food chain. Producers are the first level, primary consumers the second, secondary consumers the third, and so on. Decomposers break down dead organisms and waste.
- Energy lossesAt each level energy is lost through respiration, which releases thermal energy to the surroundings, through movement, through excretion and egestion, and because not all of the previous level is eaten or digested.
- Why food chains are shortOnly about ten per cent of the energy at one level is passed to the next, so after four or five levels there is too little energy to support another population.
- PyramidsA pyramid of numbers counts individuals and can be inverted, for example when one large tree supports many insects. A pyramid of biomass measures dry mass and is almost never inverted. A pyramid of energy is never inverted.
- Carbon cycleCarbon dioxide is removed from the air by photosynthesis and returned by respiration, decomposition and combustion. Carbon is passed along food chains as organic compounds in food.
- Population growth curveLag phase while numbers are low and organisms adjust, exponential phase of rapid growth with plentiful resources, stationary phase where births balance deaths as a limiting factor takes effect, and sometimes a death phase.
IGCSE Biology Organisms and their Environment questions and answers
4 exam-style questions written to the 0610 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Explain why only a small proportion of the energy in one trophic level is transferred to the next.
Show the worked answer
- Much of the energy taken in is released by respiration and transferred to the surroundings as thermal energy, which cannot be recovered by the next level.
- Energy is used for movement and other life processes, and is eventually lost as heat.
- Energy leaves in excretory products such as urea and in undigested material egested as faeces.
- Not all of the previous trophic level is eaten. Bones, roots and other parts are left, so their energy passes to decomposers rather than to the next consumer.
A pyramid of numbers for the food chain oak tree, insects, small birds, hawks is inverted at the base. Explain why, and explain why a pyramid of biomass for the same chain would not be inverted.
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- A pyramid of numbers counts individual organisms and ignores their size.
- A single oak tree can feed thousands of insects, so the producer bar is narrow and the primary consumer bar is very wide, giving an inverted shape at the base.
- A pyramid of biomass measures the total dry mass of organisms at each level rather than counting them.
- The oak tree has a far greater mass than all the insects feeding on it, so the producer bar is the widest and the pyramid takes the normal shape.
Name the processes that remove carbon dioxide from the atmosphere and the processes that return it, and explain how carbon in a plant can end up in the atmosphere after the plant dies.
Show the worked answer
- Photosynthesis in producers removes carbon dioxide from the atmosphere and fixes the carbon into glucose and other organic compounds.
- Respiration by plants, animals and decomposers returns carbon dioxide to the atmosphere.
- Combustion of wood and fossil fuels also returns carbon dioxide.
- When a plant dies, decomposers such as bacteria and fungi feed on the dead material.
- The decomposers respire the carbon compounds they absorb, releasing carbon dioxide back into the atmosphere. If the remains are instead compressed over millions of years they may form fossil fuels, and the carbon is returned only when those fuels are burnt.
A population of bacteria in a closed culture shows a lag phase, an exponential phase and then a stationary phase. Explain what causes the stationary phase.
Show the worked answer
- During the exponential phase resources are plentiful and reproduction is rapid, so numbers double repeatedly.
- As the population grows, nutrients in the closed culture become depleted and toxic waste products accumulate. Space also becomes limited.
- These limiting factors slow reproduction and increase the death rate.
- The stationary phase is reached when the rate of reproduction equals the rate of death, so the population size stays roughly constant.
Common mistakes in this topic
- Drawing food chain arrows in the wrong direction. Arrows show energy flow, so they point from the food to the feeder.
- Calling a producer a plant in every case. Some producers are algae or photosynthetic bacteria.
- Saying energy is recycled. Energy flows through and is lost. Nutrients are recycled.
- Confusing a pyramid of numbers with a pyramid of biomass.
- Describing decomposers as feeding on living organisms. They feed on dead material and waste.
Exam tips
- State that arrows represent the flow of energy whenever you draw or interpret a food chain.
- In energy loss questions, give at least three named routes. Vague answers lose most of the marks.
- Use dry mass when discussing biomass. Water content varies and would distort comparisons.
- For carbon cycle diagrams, label each arrow with a named process, not just a substance.
- In population curve questions, always name the specific limiting factor for that organism and habitat.
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Organisms and their Environment FAQs
Why is energy lost between trophic levels?
Energy is released as thermal energy during respiration and lost to the surroundings, used for movement and other life processes, lost in excretory products and undigested material, and left in parts of the organism that are not eaten. Only around ten per cent of the energy at one level typically reaches the next.
Why are food chains usually short?
Because so much energy is lost at each transfer, the energy available falls sharply with each level. After four or five levels there is not enough energy remaining to support a viable population of another consumer, so food chains rarely extend further.
What is the difference between a pyramid of numbers and a pyramid of biomass?
A pyramid of numbers counts the individual organisms at each trophic level regardless of their size, so it can be inverted when one large producer supports many small consumers. A pyramid of biomass measures the total dry mass at each level, so it takes account of size and is almost always the normal pyramid shape.
How does carbon return to the atmosphere?
Through respiration by plants, animals and decomposers, and through combustion of wood and fossil fuels. When an organism dies, decomposers feed on its remains and respire the carbon compounds, releasing carbon dioxide. Photosynthesis is the only major process that removes carbon dioxide from the atmosphere.
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Written to the published Cambridge IGCSE Biology (0610) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.